If your talking about the front windshield then I'm not too sure there is a "Legal Size" to be honest.
You are allowed for the cut to ingress into the roof of the car to allow for re-fueling (ie with a fuel gun)
I would say 60mm diameter would be about what we cut ours to.
If your talking about the side windows then you are allowed to cut out the two front completely, nothing out of the rear ones.
You are able to cut the rear windshield out completely too.
Regards
BM